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Significant Accounting Policies - Revenues From Contracts with Customers (Details) - USD ($) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 35,554,280 | $ 38,264,674 | $ 102,895,420 | $ 119,542,174
Investment advisory |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 33,668,351 | 35,842,083 | 97,211,253 | 111,529,134
Mutual fund administration, net |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,885,929 | 2,422,591 | 5,684,167 | 8,013,040
Diamond Hill Funds |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 23,600,801 | 26,495,289 | 69,704,006 | 84,787,789
Diamond Hill Funds | Investment advisory |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 21,714,872 | 24,072,698 | 64,019,839 | 76,774,749
Diamond Hill Funds | Mutual fund administration, net |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,885,929 | 2,422,591 | 5,684,167 | 8,013,040
Separately managed accounts, excluding performance-based fees |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 6,150,388 | 6,163,236 | 18,653,260 | 19,926,337
Separately managed accounts, excluding performance-based fees | Investment advisory |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 6,150,388 | 6,163,236 | 18,653,260 | 19,926,337
Separately managed accounts, excluding performance-based fees | Mutual fund administration, net |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 0 | 0 | 0 | 0
Performance-based fees |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,176,351 | 1,500,225 | 1,176,351 | 1,500,225
Performance-based fees | Investment advisory |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,176,351 | 1,500,225 | 1,176,351 | 1,500,225
Performance-based fees | Mutual fund administration, net |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 0 | 0 | 0 | 0
Other pooled vehicles |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,451,019 | 2,260,465 | 6,785,722 | 7,201,190
Other pooled vehicles | Investment advisory |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 2,451,019 | 2,260,465 | 6,785,722 | 7,201,190
Other pooled vehicles | Mutual fund administration, net |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 0 | 0 | 0 | 0
Model delivery |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,306,460 | 1,358,517 | 3,969,604 | 4,585,925
Model delivery | Investment advisory |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 1,306,460 | 1,358,517 | 3,969,604 | 4,585,925
Model delivery | Mutual fund administration, net |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 0 | 0 | 0 | 0
Collective investment trusts |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 869,261 | 486,942 | 2,606,477 | 1,540,708
Collective investment trusts | Investment advisory |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | 869,261 | 486,942 | 2,606,477 | 1,540,708
Collective investment trusts | Mutual fund administration, net |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Revenue | $ 0 | $ 0 | $ 0 | $ 0

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